Ammonia transport by thick ascending limb segments of rat kidney is being examined by in vitro microperfusion. The studies are designed to evaluate the contribution of this nephron segment to the regulation of renal ammonia and net acid excretion. Initial studies indicate that both cortical and medullary thick ascending limbs absorb ammonia and that ammonia absorption is blocked by luminal furosemide. Since rat thick ascending limb segments also were found to absorb bicarbonate, absorption, absorption of ammonia must occur by a mechanism other than non-ionic diffusion. Absorption of ammonia from the medullary thick ascending limb could provide a source of ammonia for its accumulation and recycling in the outer medulla.